1. Field of the Present Invention
The present invention relates generally to the field of gas detectors, and, in particular, to the art of more efficiently firing, biasing and testing xe2x80x9cheated electrodexe2x80x9d halogenated refrigerant sensors using control theory to control the operation of the detector using an advanced sensing device and one or more control loops.
2. Background Art
The need for a reliable method or apparatus for detecting leaks from refrigerant systems has long been well known. A number of leak detectors have been developed to meet this need. One well known type of leak detector makes use of a xe2x80x9cheated electrodexe2x80x9d sensing device to indicate the presence of trace quantities of halogenated refrigerants. Such sensors are generally constructed of a platinum anode/heater coil and a platinum cathode wire within the coil, and are coated with a ceramic slurry consisting of alumina and a silicate of an alkali metal such as sodium, potassium, or rubidium. The ceramic forms an electrically resistive layer between the electrodes. When heated by an electrical current passing through a first of the electrodes, an outer layer depleted of ions develops along the electrodes. When this layer is exposed to reactive gases like halogen, ions flow across the depletion zone and the conductivity of the device is increased. Thus, the presence of halogenated gases may be indicated by monitoring the current generated through the second electrode, referred to as the bias current, for a sudden increase in magnitude created by introducing the device to such gases. These sensors are commonly utilized by technicians to determine whether a refrigerant leak exists and to pinpoint its source.
For such a device to function reliably as a refrigerant sensor, the assembly must be fired (to sinter the ceramic), and also xe2x80x9cbiasedxe2x80x9d to create the ion depletion region across which ions flow in the presence of refrigerant. Typically, the firing and the biasing operations are performed separately. The firing operation takes place in a kiln at a high temperature and requires a relatively lengthy period of time. Subsequently, the fired assembly is mounted in its holder, which may be a TO-5 transistor can, and current is passed through the anode coil to heat the sensor while a bias voltage is applied between the anode coil and cathode wire. Over another relatively lengthy period of time, the depletion region is formed.
Unfortunately, prior art systems and methods have a number of significant drawbacks. First, the firing operation typically requires 3 or more hours, and the biasing operation requires up to 12 more hours, and thus collectively consume a very large amount of manufacturing time before the sensor testing for compliance to specifications may even begin. Also, the holder, such as the TO-5 transistor can, typically cannot withstand the high firing temperatures to which it must be subjected in the kiln, and thus the anode/cathode assembly must be fired separately and attached to the can afterwards, prior to biasing. Therefore, significant time and labor may have already transpired without any knowledge as to the ultimate performance (or lack there of) of the final sensor. Further, the sensor must be completely tested under operating conditions, rather than during or immediately after the biasing process, to ensure compliance to specifications, because variations in the ceramic mixture and in the construction of the sensor may have significant impact upon the final operation of that particular sensor. Finally, since the construction and the firing, biasing and testing operations require so much time, the operations must occur in large batches such that production throughput is optimized. Variations in the process could potentially produce hundreds or thousands of failures, resulting in a great deal of waste material and lost time, in addition to the time required to bring the processes back into specification.
Alternatively, some sensors are fired by passing a current through the coil to obtain temperatures sufficient to sinter the sensor. Biasing may also be accomplished by maintaining the elevated temperatures for an extended period of time. However, existing electrical heating methods involve only the crude application of a sufficient amount of heat to sinter and bias, without regard to how the temperature is raised to such temperatures, thus raising the risk of damage to the sensor during the process. No consideration is given to the amount of moisture present in the sensor as the temperature is increased. Further, such methods do not take into consideration any information about the state of the sensors being fired or biased while the heating is taking place, and thus result in significant inefficiencies in the amount of time required for the manufacture of sensors.
Thus, a need exists for an improved method of manufacturing heated electrode refrigerant sensing devices.
Briefly summarized, the present invention relates to methods and apparatuses for automatically controlling the processes of firing and biasing heated electrode refrigerant sensors for use in refrigerant leak detectors in general. Broadly defined, the present invention according to one aspect includes a method of, and apparatuses for, manufacturing a heated-electrode refrigerant sensor for use in a refrigerant detector, wherein the method includes the steps of: mounting an unfired sensor in a manufacturing station; and, while the unfired sensor remains mounted in the manufacturing station, electrically heating the sensor by applying current to the sensor, adjustably controlling the amount of power applied to the sensor to gradually increase the temperature of the sensor over a period of time, firing the sensor once the sensor reaches a suitable temperature, and, while the temperature of the sensor is elevated, biasing the sensor.
In features of this aspect, the period of time is between 4 and 30 minutes; the rate of temperature increase does not exceed 500 degrees Celsius per minute; a substantial portion of the temperature increase occurs at a generally constant rate of between 10 and 50 degrees Celsius per minute; the method further includes the step of monitoring the temperature of the sensor and the step of adjustably controlling the applied power includes adjustably controlling the applied power on the basis of the monitored temperature; the method further includes the step of providing, prior to the step of mounting, an unfired sensor having an anode/cathode assembly and a housing; the method further includes the step of attaching the housing to the anode/cathode assembly prior to mounting the unfired sensor in the manufacturing station; the steps of controlling the current to gradually increase the temperature, firing the sensor, and biasing the sensor are cumulatively completed in less than 2 hours; and the step of firing the sensor begins while the temperature is still being increased.
The present invention according to another aspect includes a method of, and apparatuses for, manufacturing a heated-electrode refrigerant sensor for use in a refrigerant detector, wherein the method includes the steps of: generating a bias current in the sensor; gradually raising the temperature of the sensor at a first rate of increase; and after substantially all moisture has been removed from the sensor, gradually raising the temperature of the sensor at a second rate of increase.
In features of this aspect, the second rate of increase is greater than the first rate of increase; the method further includes the step of determining, while raising the temperature at the first rate of increase, that substantially all moisture has been removed from the sensor; the step of determining includes monitoring the magnitude of the bias current; the step of monitoring includes detecting the substantial absence of a bias current; the step of raising the temperature of the sensor at a second rate of increase is conditional upon a determination that substantially all moisture has been removed from the sensor; the first rate of increase is between 10 and 50 degrees Celsius per minute; the second rate of increase is between 50 and 500 degrees Celsius per minute; the step of raising the temperature at a second rate of increase is initiated at a predetermined time which is between 2 and 20 minutes after the initiation of the step of raising the temperature of the sensor at the first rate of increase; the method further includes the step of biasing the sensor, after raising the temperature at a second rate of increase, by maintaining the temperature of the sensor at or above a predetermined temperature until the predetermined temperature is reached; the biasing step includes maintaining the temperature of the sensor substantially constant at the predetermined temperature for the predetermined period of time; and the method further includes firing the sensor upon reaching a suitable temperature which is reached during the gradual temperature increase at the second rate.
The present invention according to yet another aspect includes a method of, and apparatuses for, manufacturing a heated-electrode refrigerant sensor for use in a refrigerant detector, wherein the method includes the steps of: generating a bias current in the sensor; gradually increasing the temperature of the sensor from an initial temperature to a first bias temperature; upon reaching the first bias temperature, biasing the sensor by holding the temperature of the sensor generally constant for a first period of time; after the expiration of the first period of time, increasing the temperature of the sensor to a second bias temperature; and upon reaching the second bias temperature, further biasing the sensor by holding the temperature of the sensor generally constant for a second period of time.
In features of this aspect, the method further includes monitoring the bias current while the temperature is held substantially constant for the second period of time; the difference between the first and second bias temperatures is less than 50 degrees Celsius; the first period of time is predefined to be less than 6 minutes; the second period of time is predefined to be between 8 and 30 minutes; the length of the second period of time is determined on the basis of a sensor operating condition; the sensor operating condition is the magnitude of the bias current; and the variance of the sensor temperature during the first period of time and the variance of the sensor during the second period of time are each less than 5 degrees Celsius.